Grace (Brie Larson, left) reaches out to Marcus (Keith Stanfield), one of her charges at a group foster-care home, in the drama “Short Term 12.”

“Short Term 12,” opening Friday, certainly took a long time getting to theaters.

The project, set in a group home for troubled kids, began as a short film by writer-director Destin Cretton that he worked on for his graduate thesis project at San Diego State University. In 2009, it won the Jury Prize in

U.S. Short Filmmaking at Sundance (one assumes Cretton also got an A). Cretton then adapted the story into a feature script, which won a Nicholl Fellowship, given by the Academy of Motion Picture Arts and Sciences, in 2010.

Still, though, “all the while, we were trying to get financing, but it’s kind of difficult to get money for a film about at-risk teenagers and child abuse,” Cretton says. In the meantime, he made his debut feature, “I Am Not a Hipster,” and “that helped financiers see how I would tackle those things.”

Cretton had experience with “those things,” having worked for two years at a foster-care home similar to the one in “Short Term 12.”

“I was going to be the good guy,” Cretton says of his stint at the home. “I was going to change things and be great for these kids. And within the first hour, a kid threw a plastic chair through a plate-glass window 2 feet from my head, and I was snapped into the reality of how complicated things were.”

The central character in the film is Grace (Brie Larson), a twentysomething worker at the facility. Her typical day includes sprinting after a kid making a break for it, preparing another who’s aging out of the system, and otherwise trying to stay on top of the needs of kids who have seen the worst of what the world can throw at them.

A woman working against the odds to help the kids in her care — sounds like something you’d see on Lifetime, right?

“That was my biggest fear,” Cretton says. “There was never a moment when I wasn’t deathly afraid of the story being tipped over to the overly sentimental, easy-fix type of scenario. Every decision was trying to make sure the world felt as complicated as it was when I was working there.”

Cretton says he tried to make sure that the emotions and situations in the film were presented realistically. Sometimes, though, reality proved too strange for fiction.

“There were a lot of stories I heard where I literally said, ‘I can’t put that in the movie because no one would believe it.’”